1. Field of the Invention
The present invention relates to a hydraulic control system for heavy construction equipment, and more particularly to a hydraulic control system that can minimize the flow rate of a hydraulic fluid being discharged from a variable displacement hydraulic pump by using pilot pressure constantly produced by a pilot pump when a switching valve is in a neutral position, and can adjust the flow rate of the hydraulic fluid being discharged from the variable displacement hydraulic pump by using pressure produced by a pressure generator positioned at the most downstream side of a bypass passage if a separate input signal is applied to the pressure generator when the switching valve is operated.
2. Description of the Prior Art
FIG. 1 shows a hydraulic circuit diagram illustrating the construction of a conventional hydraulic control system with negative control.
Referring to FIG. 1, the conventional hydraulic control system includes a main variable displacement hydraulic pump 2, a plurality of actuators (not shown), and a plurality of switching valves 10, 12, and 14 installed in series between the main variable displacement hydraulic pump 2 and a plurality of the actuators.
A pressure generator 30 is installed at the most downstream side of a bypass passage 20, and pressure produced by the pressure generator 30 is fed to a flow control valve for the hydraulic pump via a pressure signal line 32 to control the flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump 2 in response to the pressure.
With the construction of the conventional hydraulic system, when the switching valves 10, 12, and 14 are in a neutral mode, a hydraulic fluid flowing through the bypass passage 20 increases pressure by a specific level in the pressure signal line 32 through the pressure generator 30. The pressure is applied to the flow control device 40 for the main variable displacement hydraulic pump, so that the flow control device 40 decreases the flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump 2.
The hydraulic control system has been widely used for its convenient manipulation of a hydraulic excavator. This is because the pressure of the hydraulic fluid fed back to the main variable displacement hydraulic pump 2 from the switching valves 10, 12, and 14 is decreased, or the hydraulic fluid being discharged from the main variable displacement hydraulic pump 2 is supplied to the actuator, with a part of the hydraulic fluid draining away.
In this case, a part of the hydraulic fluid supplied from the main variable displacement hydraulic pump 2 to the switching valves 10, 12, and 14 drains away to a tank T via the bypass passage 20, when the switching valves 10, 12, and 14 are in a neutral mode or is in an operation mode. Consequently, since energy corresponding to the drained part is converted into heat, it creates a problem of energy loss.
More particularly, the pressure generated by the pressure generator 30 is fed to the flow control device 40 via the pressure signal line 32 according to the motion of the switching valves 10, 12, and 14. When the switching valves 10, 12, and 14 are in the neutral mode, the pressure in the pressure signal line 32 is raised, and thus the flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump 2 is decreased. If the switching valves 10, 12, and 14 move, the bypass passage 20 is closed. Thus, the pressure in the pressure signal line is lowered, and the flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump 2 is increased. Therefore, it will be understood from the pump pressure diagram shown in FIG. 2 that the pressure of the main variable displacement hydraulic pump 2 is increased by the load applied to the actuator connected to the switching valves 10, 12, and 14.
When the switching valves 10, 12, and 14 are in the neutral mode, the pressure (e.g., of about 30 to 40 bars) is generated corresponding to the pressure in the pressure signal line 32 by the pressure generator 30 in order to minimize the flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump 2. The pressure drains away to the tank T via the bypass passage 20, which is not effective in view of energy efficiency.
As shown in FIG. 3, another conventional hydraulic control system includes a main variable displacement hydraulic pump 52 connected to a hydraulic pressure supply passage 50, a plurality of actuators (not shown) driven by the hydraulic fluid discharged from the main variable displacement hydraulic pump 52, switching valves 60 and 62 interposed between the main variable displacement hydraulic pump 52 and the actuators, and connected in parallel with the hydraulic pressure supply passage 50, first flow control devices 64 and 66 interposed between the switching valves 60 and 62 and the actuators, a load pressure signal passage 70 for guiding a part of the hydraulic fluid, which is supplied by a switching motion of the switching valves 60 and 62, to a tank T via the first flow control devices 64 and 66, a second flow control device 82 installed on one side of the bypass passage 80 branched from the hydraulic pressure supply passage 50, and operated in an open direction or a closed direction according to the pressure difference between the pressure in the load pressure signal passage 70, pressure of a spring, and pressure in the bypass passage 80 to adjust the flow rate of the hydraulic fluid passing through the bypass passage 80, a pressure generator 90 installed at the most downstream side of the bypass passage 80 for generating pressure, a pressure signal line 92 pressurized by the pressure generator 90, and a flow control device 94 controlling the flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump by adjusting the inclination angle of a swash plate in the main variable displacement hydraulic pump 52.
With the construction shown in FIG. 3, when the switching valves 60 and 62 operate, the flow rate of the hydraulic fluid passing through the second flow control device 82 is varied depending upon the load pressure in the load pressure signal passage 70 and the pressure in the bypass passage 80. The flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump 52 is controlled by variation of the pressure in the pressure signal line 92. When the switching valves 60 and 62 are in the neutral mode, the pressure corresponding to the pressure applied in the pressure signal line 92 by the pressure generator 90 is generated in the main variable displacement hydraulic pump 52 so as to minimize the flow rate of the hydraulic fluid being discharged from the main variable displacement hydraulic pump 52. The pressure drains away to the tank T via the bypass passage 80, which is still not effective in view of energy efficiency.